State of the art hearing instruments are usually either behind-the-ear (BTE) hearing instruments, in-the-ear (ITE) hearing instruments, in-the-canal (ITC) hearing devices or completely-in-the-canal (CIC) hearing instruments. ITE, and especially ITC and CIC hearing instruments are less visible than BTE hearing instruments and are therefore preferred by many users. However, in these devices the space in the ear canal has to be used efficiently, and the ear canal essentially has to be closed by the device so as to minimize acoustic feedback due to the proximity of the sound outlet of the receiver and the sound inlet of the microphone. This plugging of the ear canal may cause undesirable effects, known as occlusion effect which has an impact on the perception of the wearer's own voice and on the wearing comfort. The occlusion effect may also occur when BTE hearing instruments are used, since also BTE hearing instruments comprise a piece (“earpiece”) placed in the ear canal, which is used for holding sound conduction tube(s) and/or other elements.
In this text, the term “earpiece” or “in-the-ear-canal component” is used to denote any device or device part of a hearing instrument that is meant to be placed at least partially in the ear canal of the user. It may for example be a ITE, ITC, or CIC hearing instrument. It may as an alternative be an earpiece (or otoplastic) of a hearing instrument which also comprises an outside-the-ear-canal component, for example a behind-the-ear component of a BTE. In the case of a BTE with an open fitting, the in-the-ear-canal component may merely be a fixation means for at least one sound tube. Concerning different types of in-the-ear-canal components and ways to connect them to a possible outside-the-ear-canal component, it is referred to the European patent application publication EP 1 681 904, the teaching of which is incorporated herein by reference.
In order to reduce the occlusion effect, in-the-ear-canal components comprising a “vent”—a duct through the in-the-ear-canal component—are used. Hearing instruments with large vents are especially popular, since the open fitting is perceived as very comfortable by the user. One of the reasons for this is that the occlusion effect is greatly reduced, and the own voice is perceived more naturally. However, large vents also have disadvantages.    a. Strong direct sound through the vent, which may not be controlled by the hearing instrument and which, due to delay differences, may interfere with the sound produced by the hearing instrument receiver.    b. Especially in ITE, ITC, and CIC hearing instruments, enhanced tendency for feedback, since the sound produced by the hearing instruments gets through the vent back to the microphone without substantial attenuation.    c. Reduced space: The space used up by the vent diminishes the design degrees of freedom, for example concerning the placement of a receiver in the instrument.
There are several proposals for dealing with sound conduction through ducts. The active control in ducts was proposed, for example in U.S. Pat. No. 4,473,906, to reduce noise carried through heating and ventilating ducts in factories and the like. Such systems rely on a first microphone placed along the duct which detects a noise signal, and a loudspeaker arranged downstream of the microphone that produces a compensation signal. In more advanced embodiments, there may be a second microphone for detecting a remaining signal to be minimized. Such active control systems, however, are as such not suitable for hearing instruments, since in hearing instruments the sound conduction tube is extremely short, and in hearing instruments sound conduction not only in one but in both directions is an issue.
Active direct sound compensation systems aim at the canceling of direct sound transmitted towards the eardrum, especially as active ear protection devices. Such active direct sound compensation systems have been described in the publications EP 1 499 159, U.S. Pat. No. 6,445,799, U.S. Pat. No. 5,740,258, and WO 2005/052911. EP 1 499 159, U.S. Pat. No. 6,445,799, and U.S. Pat. No. 5,740,258 describe combinations of a microphone with a loudspeaker that rely on the principle of evaluating, from the microphone signal, an inverse noise and radiating the inverse noise by the loudspeaker downstream of the microphone. WO 2005/052911 discloses a further example of such a feed forward noise canceller, which is suitable for attenuating sound signals bypassing the hearing instrument in situations where an auxiliary signal is received, for example through a wireless or wired connection, such as a telecoil. The wanted signal is fed to a receiver from an electrical input—such as a telecoil—whereas the signal of a microphone is fed to a noise cancellation part, which supplies a canceling signal to a receiver in the ear canal. An error microphone is used for adjusting the attenuating signal.
There are known disadvantages of such approaches. Firstly, no receiver is available with sufficient power for providing the compensation signal (inverse noise). Secondly, in practice it is difficult to separate between the wanted signal and the disturbing signal to be reduced. Thirdly, the electro-acoustic transfer function in the excitation path (often mentioned as error path) is unfavorable with respect to the needed compensation filter. Finally, these approaches do not provide a solution to the problem of enhanced feedback due to there being a vent.
A further category of active systems for hearing instruments, therefore, deals with hearing instruments with no vent or only a small vent and with an active reduction of the occlusion effect. The disclosures of WO 2004/021740 and U.S. Pat. No. 6,937,738 are examples of such systems.
Yet a further category of systems deals with an active canceling of feedback through a vent. In U.S. Pat. No. 5,033,090 a microphone signal in the vent is used to improve the feedback tendency by way of suitable subtraction from the input signal. The vent microphone features the advantage that the unknown feedback path from the receiver to the vent microphone is taken into account. The remaining path from the vent microphone to the input microphone, however, is hard to estimate with sufficient accuracy, since the sound radiation from the vent to the concha scatters strongly from individual to individual. Thus, in practice, the estimation is difficult. Therefore, one may as well estimate the whole feedback path from the receiver to the input microphone. This is done in modern feedback cancellers. Such feedback cancellers, however, today have reached their limits in terms of impact and artifacts.
The European patent publication EP 1 499 159 describes, next to the above-mentioned cancellation of direct sound by means of a microphone and a receiver also the canceling of sound from the opposite direction. To this end, the sound is recorded by the microphone in the vent and is, via a compensation device, radiated at a different position in the vent. This disclosure thus relies on the same principle as above-described, and entails the same disadvantages. It features the additional disadvantage that two microphones have to be placed adjacent the vent.